<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<style>
   .yeye{
       width: 400px;
       height: 250px;
       background: olive;
       padding: 20px;   
       position: relative; /* 爷爷定位，作为参照物 */
   }
   .father{
       width: 300px;
       height: 200px;
       background: black;
       padding: 20px;
       /*position: relative;*/  /* 爸爸定位 */
   }
   .a,.b,.c{
       width: 200px;
       height: 60px;
  }
  .a{
      background: palevioletred;
  }
  .b{
      background: darkcyan;
      position: absolute;
      top: 30px;
      left: 50px;
  }
  .c{
      background: chocolate;
  }
</style>

<body>
        <div class="yeye"> 
            <div class="father"> 
                <div class="a">1</div> 
                <div class="b">2</div> 
                <div class="c">3</div> 
            </div> 
        </div>
</body>
</html>